1. Field of the Invention
The present invention relates to an optical module in which an optical signal transmitter and an optical signal receiver are integrated.
2. Description of the Related Art
The Internet traffic using the internet protocol (IP), which has spread rapidly in recent years, offers large-capacity transmission services by employing an optical fiber network for a transmission line.
Meantime, an optical transceiver for transmitting and receiving optical signals has been reduced in size as the mounting density of the device has been becoming higher and higher. Such an optical transceiver has a problem of crosstalk in which a signal passing through a transmitter leaks into a receiver adjacent to the transmitter to produce noises, etc. Some techniques have been disclosed as measures against crosstalk, which include a technique of attaching an electric wave absorber to crosstalk electric wave generation spot on a receiver and to a circuit portion showing high sensitivity on a receiver (for example, Japanese Patent Application Laid-Open Publication No. 2002-185408), and a technique of providing a transmitter with a low-pass filter (for example, International Publication Pamphlet No. 00/33490).
A ferrite bead is used as an element that is capable of suppressing crosstalk in a frequency band handled by optical communication transceivers currently available. This ferrite bead is provided in such a manner that a ferrite is formed into a small cylindrical shape, etc., and is interposed to a signal path. The ferrite bead works on such an operating principle that the ferrite absorbs a high-frequency noise component out of components contained in a passing signal, and transforms the noise component into heat to eliminate the noise component. Some techniques utilizing this principle have been disclosed. Such techniques include a technique of attaching a ferrite bead to a cable connected to a communication device (for example, Japanese Patent Application Laid-Open Publication No. H10-208818), and a technique of arranging a ferrite bead to a ground line of each device (for example, Japanese Patent Application Laid-Open Publication No. H10-209664).
The major cause of crosstalk resulting in a signal leak into an adjacent line has been thought because of a phenomenon that a high-power signal passing through a transmitter propagates through space to interfere with a signal passing through a receiver. FIG. 9 depicts an example of a countermeasure against crosstalk in a conventional optical transceiver. A transceiver 10 has a single printed substrate 11 that carries a transmitter 12 including a light-emitting diode driving circuit (LD driving circuit) driving an LD serving as a light-emitting element, and a receiver 13 including a main amplifier circuit. The transmitter 12 is connected to a light-emitting unit 14 including the LD, and the receiver 13 is connected to a light-receiving unit 15 including a PD. The light-emitting unit 14 and the light-receiving unit 15 are separated across a gap of several millimeters long. The transceiver 10 that is miniaturized in such a manner is protected from crosstalk caused by spatial signal propagation, by covering the receiver 13 with a shield 16. A shield panel or a shield case is used as the shield 16.
Being incorporated in a single unit of the miniaturized transceiver 10, the transmitter 12 and receiver 13 are supplied with power through the same power line, and are grounded through the same ground line. This produces common impedance, because of which the operation of the high-power transmitter 12 exercises an effect on the receiver 13. The effect of the common impedance has also been considered to be one of the causes of crosstalk. Thus, separating the transmitter 12 from the receiver 13 by providing the shield 16 shown in FIG. 9 has been considered to be effective in reducing crosstalk caused by the common impedance.
In these days, however, such common specifications as small form-factor pluggable (SFP), small form factor (SFF), and 10-gigabit small form factor pluggable (XFP) have been established as standardized optical modules for use in optical transceivers operated in the Internet traffic using an optical fiber network. Hence the functions and forms of optical transceivers have been more and more integrated.
As a result, providing an optical module with the shield 16 is difficult when the optical module is under a size restriction. If a shield panel or a shield case is assembled into the shield 16, and if the receiver 13 is housed in the shield 16, the overall size of the optical module cannot be reduced into a compact size.